Field of the Inventions
The disclosed inventions relate generally to integrated circuit fabrication, techniques for fabrication of computer memory, and contact formation therefor.
Description of the Related Art
As a consequence of many factors, including demands for increased portability, computing power, memory capacity, and energy efficiency in modern electronics, integrated circuits are continuously being reduced in size. To facilitate these size reductions, the sizes of the constituent features, such as electrical devices and interconnect line widths, that form the integrated circuits, are also constantly being decreased.
The trend of decreasing feature size is most evident in memory circuits or devices, such as dynamic random access memories (DRAMs), static random access memories (SRAMs), ferroelectric (FE) memories, etc. To take one example, DRAM typically comprises millions of identical circuit elements, known as memory cells.
By decreasing the sizes of constituent electrical devices and the conducting lines that access them, the sizes of the memory devices incorporating these features can be decreased. Storage capacities for a given chip area can thus be increased by fitting more memory cells onto memory devices without increasing the overall size of the devices.
The continual reduction in feature size places ever greater demands on the techniques used to form the features. One well-known technique is photolithography, commonly used to pattern features, such as conductive lines, on a substrate. The concept of pitch can be used to describe the size of these features. For the repeating patterns typical of memory arrays, pitch is defined as the distance between an identical point in two neighboring features. Adjacent features are typically separated by a material, such as an insulator. As a result, pitch can be viewed as the sum of the width of the feature and of the width of the space or material separating that feature from a neighboring feature. Due to optical factors, such as lens limitations and light or radiation wavelength, photolithographic techniques have minimum pitches below which a particular photolithographic technique cannot reliably form features. This minimum pitch is commonly referred to by a variable defining one half of the minimum pitch, or feature size F. This variable is often referred to as a “resolution.” The minimum pitch definable by photolithography, 2F, places a theoretical limit on feature size reduction.
One method for improving the density possible using conventional photolithographic techniques is to change the layout of a memory device in order to fit more memory cells in the same area without changing the pitch. Using such a method, the size of the memory device can be reduced without exceeding the minimum pitch, 2F, dictated by optical limitations. Alternatively, the memory device may be configured to hold more memory cells, while maintaining a constant pitch.
Memory layout changes, particularly those accompanied by increased feature density, and other factors have contributed to the need for improved subcomponent configurations and methods for forming subcomponents that are adapted to the memory layout changes.